Liquid-assisted cryogenic cleaning

ABSTRACT

The present invention is directed to the use of a high vapor pressure liquid prior to or simultaneous with cryogenic cleaning to remove contaminants from the surface of substrates requiring precision cleaning such as semiconductors, metal films, or dielectric films. A liquid suitable for use in the present invention preferably has a vapor pressure above 5 kPa and a freezing point below −50° C.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application claims the benefit of priority under 35 U.S.C.Section 119(e) of U.S. Provisional Ser. No. 60/369,853, filed Apr. 5,2002, which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to the use of a liquid, eithersimultaneously or sequentially, with cryogenic cleaning to aid in theremoval of foreign materials, e.g. particles and other contaminants,from semiconductor surfaces, metal films, dielectric films, and othersurfaces requiring precision cleaning.

BACKGROUND OF THE INVENTION

Cleaning or surface preparation of silicon wafers with or withoutvarious layers of films is very critical in integrated circuitmanufacturing processes. The removal of particles and contaminants fromwafer surfaces is performed at several critical process steps during thefabrication of integrated circuits. At a 0.18 μm technology node, 80 outof 400 steps or 20% of the fabrication sequence is dedicated tocleaning. The challenges of cleaning technology are multiplied by thevaried types of films, topographies, and contaminants to be removed infront-end-of-line (FEOL) and back-end-of-line (BEOL) cleaning processes.Removal of particles is an important part of this cleaning.

For the defect-free manufacture of integrated circuits, theInternational Technology Roadmap for Semiconductors (ITRS) indicatesthat the critical particle size is half of a DRAM ½ pitch [1]. Thus, atthe 130 nm technology node, the DRAM ½ pitch being 130 nm, the criticalparticle size is 65 nm. Therefore, particles larger than 65 nm size mustbe removed to ensure a defect-free device.

Such small particles are difficult to remove since the ratio of theforce of adhesion to removal increases for smaller-sized particles. Forsubmicron particles, the primary force of adhesion of the particles to asurface is the Van der Waals force. This force depends on the size ofthe particle, the distance of the particle to the substrate surface, andthe Hamaker constant. The Van der Waals force for a sphericalparticulate on a flat substrate is given as in equation 1:$\begin{matrix}{F_{ad} = \frac{A_{132}d_{p}}{12Z_{0}^{2}}} & (1)\end{matrix}$where A₁₃₂ is the Hamaker constant of the system composed of theparticle, the surface and the intervening medium; d_(p) is the particlediameter; and Z₀ is the distance of the particle from the surface. TheHamaker constant A₁₃₂ for the composite system is given as in equation(2)A ₁₃₂ =A ₁₂ +A ₃₃ −A ₁₃ −A ₂₃   (2)

The relationship of the Hamaker constant of two dissimilar materials isexpressed as the geometric mean of the individual Hamaker constants asA_(ij)=(A_(ii)*A_(jj))^(1/2) where A_(ii) and A_(jj) are the Hamakerconstants of materials i and j. It is calculated theoretically usingeither the Lifshitz or the London models. The Hamaker constant forparticles and surfaces used in integrated circuit manufacturingprocesses is given in literature [2, 3] and is less when the interveningmedium is liquid as compared to air. The Van der Waals force, beingdirectly proportional to the Hamaker constant, is therefore reduced whenthere is a liquid layer between the particle and the surface.

In addition to the difficulty in removing small particles from thesurface, there are various types of organic and metal-organiccontaminants which must be cleaned away. As an example, etching is donein integrated circuit device fabrication processes at a number of stepsboth in FEOL and BEOL to form patterns. The etch is often performed byreactive ion etching (RIE) which generally has a physical and a chemicalcomponent to it. Following this process, the etch residues, which arepolymeric sometimes with metallic contaminants embedded inside thepolymeric matrix, have to be removed. The photoresist film left behindafter the etching also has to be removed prior to the next step in theintegrated device fabrication process. In case of chemical-mechanicalpolishing, the polishing steps may use Cerria, alumina or silicaslurries. After polishing, the slurry and any residues from the slurryadditives need to be cleaned from the wafer surface before the nextlayer of film is deposited. Thus, there is a wide variety of residues,particles and other foreign materials which need to be cleaned both fromthe surface of the wafer as well as inside any etched features.

The prior art processes use CO₂ or argon cryogenic sprays for removingforeign materials from surfaces. As examples, see U.S. Pat. No.5,931,721 entitled Aerosol Surface Processing; U.S. Pat. No. 6,036,581entitled Substrate Cleaning Method and Apparatus; U.S. Pat. No.5,853,962 entitled Photoresist and Redeposition Removal Using CarbonDioxide Jet Spray; U.S. Pat. No. 6,203,406 entitled Aerosol SurfaceProcessing; and U.S. Pat. No. 5,775,127 entitled High Dispersion CarbonDioxide Snow Apparatus. In all of the above prior art patents, theforeign material is removed by physical force involving momentumtransfer to the contaminants where the intervening medium betweenparticle and substrate surface is air. Since the force of adhesionbetween the contaminant particles and the substrate is strong, the priorart processes are ineffective for removing small, <0.3 μm, particles.

U.S. Pat. No. 6,332,470, entitled Aerosol Substrate Cleaner, disclosesthe use of vapor only or vapor in conjunction with high pressure liquiddroplets for cleaning semiconductor substrate. Unfortunately, the liquidimpact does not have sufficient momentum transfer capability as solidCO₂ and will therefore not be as effective in removing the smaller-sizedparticles. U.S. Pat. No. 5,908,510, entitled Residue Removal bySupercritical Fluids, discloses the use of cryogenic aerosol inconjunction with Supercritical fluid or liquid CO₂. Since CO₂ is anon-polar molecule, the solvation capability of polar foreign materialis significantly reduced. Also, since the liquid or Supercritical CO₂formation requires high pressure (greater than 75 psi for liquid and1080psi for Supercritical), the equipment is expensive.

As such, there remains a need for a more efficient and effective removalprocess of contaminants, including particles, foreign materials, andchemical residues, from the surfaces of substrates such as semiconductorwafers, metal films, dielectric films, and other substrates requiringprecision cleaning.

SUMMARY OF THE INVENTION

The present invention provides for a new and improved process for thecleaning of substrate surfaces such as semiconductors and metal anddielectric films to remove contaminants.

The invention uses a high-vapor pressure liquid prior to cryogeniccleaning to reduce the Van der Waals force of adhesion of the foreignmaterial on the surface. The liquid is sprayed onto the surface andpreferably covers the surface for a short period of time. Preferably,the liquid covers the surface for at least one minute. Following thiswetting period, the cryogenic spray is initiated. The presence of theliquid will reduce the force of adhesion of the contaminants on thesurface thereby making it easier for the particles from the cryogenicspray to dislodge the contaminants from the surface. The liquid may alsoremove the bulk water from the surface prior to cryogenic cleaning, suchas is used in co-pending U.S. patent application Ser. No. 10/215,859filed on Aug. 9, 2002and entitled Post CMP Cleaning Using a Combinationof Aqueous and Cryogenic Cleaning. The liquid, if chosen with thecorrect properties, may also dissolve organic contaminants from thesubstrate surface. The high vapor pressure liquid may be appliedsimultaneously with the cryogenic cleaning.

BRIEF DESCRIPTION OF THE FIGURES

Embodiments of the present invention are described with reference to theFigures in which:

FIG. 1 is a schematic diagram of the apparatus used in standard CO₂cryogenic cleaning; and

FIG. 2 is a graph showing the efficiency of particle removal compared toparticle size for both standard cryogenic cleaning and the presentliquid-assisted cleaning process.

DETAILED DESCRIPTION

The invention uses liquids having high vapor pressure to reduce the Vander Waals force between foreign material and a substrate surface such asa semiconductor wafer surface or film surface. The high vapor pressureliquid is sprayed on to the surface of the substrate. It is followedwith cryogenic cleaning. The initial spraying of liquid will reduce theVan der Waals forces thereby allowing the cryogenic cleaning to moreeasily remove foreign material from the substrate surface. If theupstream process prior to the cryogenic cleaning is an aqueous basedprocess, as in co-pending U.S. patent application Ser. No. 10/215,859,then the liquid may also remove the bulk water prior to the cryogeniccleaning. Further, the high vapor pressure liquid may act to dissolveorganic contaminants from the surface. A particular high-vapour pressureliquid will be chosen depending on the organic contaminants contained onthe substrate surface. A skilled person in this field will be aware ofthe types of liquids which would dissolve common organic contaminants.

The liquids suitable for use in the present invention have high vaporpressures. Liquids which are suitable for use include, but are notlimited to, ethanol, acetone, ethanol-acetone mixtures, isopropylalcohol, methanol, methyl formate, methyl iodide, ethyl bromide,acetonitrile, ethyl chloride, pyrrolidine, and tetrahydrofuran. However,any liquid having a high vapor pressure may be used. High vapor pressureliquids will readily evaporate off the surface of the substrate withoutthe need for drying by heating or spinning the substrate. The liquidsalso preferably have low freezing points and are polar in nature. Thelow freezing point of the liquids ensures that any residual liquid lefton the wafer surface at the time of cryogenic cleaning will not freezedue to the drop in wafer temperature than can be attained during thecryogenic cleaning process. The polarity of the liquid aids in thedissolution of the organic and inorganic contaminants on the wafersurface. Preferably, the vapor pressure of the liquid is greater than orequal to 5 kPa at 25° C., the freezing point of the liquid is less thanor equal to about −50° C., and the dipole moment is greater than about1.5 D.

This process may be used on any substrate surface requiring precisioncleaning. These surfaces include semiconductor surfaces as well as metaland dielectric films. Therefore, whenever the term “semiconductor”,“metal film ”, “dielectric film”, or “wafer” is used herein, it isintended that the same process may be applied to other substratesurfaces. Other surfaces include hard disk media, optics, GaAssubstrates and films in compound semiconductor manufacturing processes.Examples provided herein are not meant to limit the present invention.

In one embodiment of the present invention, the high-vapor pressureliquid is sprayed onto the surface of a semiconductor wafer at atemperature of 30°-50° C. The liquid may be sprayed either as a thickfilm or as a thin layer. The layer is preferably at least 5-10 Å thick.It is preferably sprayed using a misting nozzle made of Teflon used inwet benches for spraying deionized water onto wafer surfaces. However,any other nozzle used in the art may be employed. The wafer ispreferably covered with the liquid for at least one minute andpreferably up to 10 minutes. The liquid may be applied onto the surfaceonce during this time period or it may be sprayed multiple times toensure that the wafer surface remains wet. As well, the wafer may berotated at approximately 100 rpm while the liquid is sprayed onto it toensure uniform coverage of the wafer surface.

Following this wetting period, the CO₂ cryogenic spraying is initiated.Cryogenic spraying processes may use carbon dioxide, argon or othergases and are well known within the art. Any known technique may beused. The result of the initial high vapor pressure liquid applicationis the reduction of the Hamaker constant and hence the Van der Waalsforces. This application lowers the forces of adhesion of the foreignmaterial to the wafer surface and the foreign material is easier toremove from the wafer surface than through the use of only cryogeniccleaning. It also removes bulk water in a prior aqueous cleaningprocess.

A standard CO₂ cryogenic cleaning process is described in U.S. Pat. No.5,853,962, which is incorporated herein by reference. As an example of atypical CO₂ cryogenic cleaning system, reference is made to the system11 shown in FIG. 1. This system comprises a cleaning container 12 inwhich system gases are circulated in the general direction indicated bythe arrows in FIG. 1. The cleaning container 12 provides an ultra clean,enclosed or sealed cleaning zone. Ultra cleanliness of the cleaning zonemay be achieved by virtue of means, such as a blower motor 7, forpassing system gases through an ultra purification filter, such as anultra low particulate air (ULPA) filter 6, as shown in FIG. 1. A wafer 1is held on a platen 2 by vacuum within the cleaning zone. The platen 2beneath the wafer 1 is kept at a controlled temperature of up to 100° C.Liquid CO₂, from a cylinder at room temperature and 850 psi, is firstpassed through a sintered in-line filter 4 to filter out very smallparticles from the liquid stream to render the carbon dioxide as pure aspossible and to reduce contaminants in the stream. The liquid CO₂ isthen made to expand through a small aperture nozzle 3, preferably offrom 0.05 to 0.15″ in diameter. The rapid expansion of the liquid causesthe temperature to drop, resulting in the formation of solid CO₂ snowparticles entrained in a gaseous CO₂ stream flowing at a rate ofapproximately 1-3 cubic feet per minute. The stream of solid and gaseousCO₂ is directed at the wafer surface at an angle of about 30° to about60°, preferably at an angle of about 45°. The nozzle is preferablypositioned at a distance of approximately 0.375″ to 0.5″ measured alongthe line of sight of the nozzle to the wafer surface. During thecleaning process, the platen 2 moves back and forth on track 9 in the ydirection while the arm 8 of the cleaning nozzle moves linearly on thetrack 10 in the x direction. This results in a rastered cleaning patternon the wafer surface of which the step size and scan rate can be pre-setas desired. The humidity in the cleaning chamber is preferablymaintained as low as possible, for example, <−40° C. dew point. The lowhumidity is present to prevent the condensation arid freezing of thewater on the wafer surface from the atmosphere during the cleaningprocess which would increase the force of adhesion between thecontaminant particles and the wafer surface by forming crystallinebridges between them. The low humidity can be maintained by the flow ofnitrogen of clean dry air.

As well, throughout the cleaning process, it is important that theelectrostatic charge in the cleaning chamber be neutralized. This isdone by the bipolar corona ionization bar 5. The system also has apolonium nozzle mounted directly behind the CO₂ nozzle for enhancing thecharge neutralization of the wafer which is mounted on an electricallygrounded platen. The electrostatic charge develops bytriboelectrification due to the flow of CO₂ through the nozzle andacross the wafer surface and is aided by the low humidity maintained inthe cleaning chamber.

It is desirable to remove particulate contaminants, such as particulatecontaminants that are submicron in size, such as less than or equal toabout 0.76 μm in size, from the wafer surface. For particularcontaminants, the removal mechanism is primarily by momentum transfer ofthe CO₂ cryogenic particles to overcome the force of adhesion of thecontaminant particles on the wafer surface. Once the particles are“loosened”, the drag force of the gaseous CO₂ removes them from thesurface of the wafer. It is also desirable to remove organic filmcontaminants from the wafer surface. The cleaning mechanism for organicfilm contaminants is by the formation of a thin layer of liquid CO₂ atthe interface of the organic contaminant and the surface due to theimpact pressure of the cryogenic CO₂ on the wafer surface. The liquid COcan then dissolve the organic contaminants and carry them away from thewafer surface.

Alternatively, the liquid can be applied simultaneously with the CO₂cryogenic cleaning. In such a case, a second nozzle for spraying theliquid would be mounted in conjunction with a first nozzle used for CO₂cryogenic cleaning. The liquid would preferably be applied in a thinlayer and the CO₂ cryogenic cleaning would continue simultaneously withthe spraying of the liquid onto the substrate.

As a result of the use of the high vapor pressure liquid, the removal ofparticle contaminants by cryogenic cleaning is significantly improved.FIG. 2 shows the efficiency of particle removal compared to particlesize for both standard cryogenic cleaning as well as liquid-assistedcryogenic cleaning. Removal of particles having a size below 0.76 μm issignificantly improved with the use of the present liquid assisted CO₂cryogenic cleaning process rather than standard CO₂ cryogenic cleaning.For particle sizes ranging from 0.98 μm to 2.50 μm, there was nosignificant difference in the removal of particles between the use ofthe present liquid assisted cryogenic cleaning and the standard CO₂cryogenic cleaning process.

The embodiments and examples of the present application are meant to beillustrative of the present invention and not limiting. Otherembodiments which could be used in the present process would be readilyapparent to a skilled person. It is intended that such embodiments areencompassed within the scope of the present invention.

REFERENCES

-   [1]. International Technology Roadmap for Semiconductors 2001    Edition.-   [2]. Handbook of Semiconductor Wafer Cleaning Technology Science,    Technology and Applications, Edited by Werner Kern, Noyes    Publications, 1993.-   [3]. Particle Control for Semiconductor Manufacturing, Edited    by R. P. Donovan, Marcel Dekker Inc., 1990.

1. A method for removing at least one contaminant from a surface,comprising: applying at least one liquid of high vapor pressure to thesurface, the at least one liquid sufficient for reacting with the atleast one contaminant on the surface; and applying cryogenic material tothe surface.
 2. A method for removing at least one contaminant from asurface, comprising: applying at least one liquid of high vapor pressureto the surface, the at least one liquid sufficient for reacting with theat least one contaminant on the surface; and applying cryogenic materialto the surface at low humidity.
 3. The method of any one of claims 1 and2 wherein said applying at least one liquid and said applying cryogenicmaterial occur simultaneously.
 4. The method of any one of claims 1 and2, wherein said applying at least one liquid and said applying cryogenicmaterial occur sequentially.
 5. The method of any one of claims 1 and 2,wherein said applying at least one liquid precedes said applyingcryogenic material.
 6. The method of any one of claims 1 and 2, whereinthe at least one liquid has a vapor pressure greater than or equal to 5kPa at 25° C.
 7. The method of any one of claims 1 and 2, wherein the atleast one liquid has a freezing point of less than or equal to about−50° C.
 8. The method of any one of claims 1 and 2, wherein the at leastone liquid has a dipole moment of greater than about 1.5 D.
 9. Themethod of any one of claims 1 and 2, wherein the at least one liquid isselected from a group consisting of ethanol, acetone, ethanol-acetonemixtures, isopropyl alcohol, methanol, methyl formate, methyl iodide,ethyl bromide, acetonitrile, ethyl chloride, pyrrolidine,tetrahydrofuran, and any combination thereof.
 10. The method of any oneof claims 1 and 2, wherein said applying the at least one liquidcomprises applying the at least one liquid in a layer of at least 5 Å.11. The method of any one of claims 1 and 2, wherein said applying theat least one liquid comprises allowing the at least one liquid tocontact the surface for at least one minute.
 12. The method of any oneof claims 1 and 2, wherein during said applying the at least one liquid,the surface is rotated.
 13. The method of any one of claims 1 and 2,wherein said applying the at least one liquid comprises removing bulkwater from the surface.
 14. The method of any one of claims 1 and 2,wherein said applying the at least one liquid comprises dissolving theat least one contaminant.
 15. The method of any one of claims 1 and 2,wherein said applying cryogenic material comprises directing a stream ofgaseous and particulate cryogenic material at the surface.
 16. Themethod of any one of claims 1 and 2, wherein the cryogenic materialcomprises at least one gas.
 17. The method of any one of claims 1 and 2,wherein said applying cryogenic material comprises physically cleaningthe surface at low humidity.
 18. The method of any one of claims 1 and2, wherein said applying cryogenic material comprises physicallycleaning the surface at a dew point temperature of less than −40° C. 19.The method of any one of claims 1 and 2, wherein the surface is at atemperature of up to 100° C.
 20. The method of any one of claims 1 and2, wherein the surface is at a temperature of 30° C. to 50° C.
 21. Themethod of any one of claims 1 and 2, wherein the at least onecontaminant is a particulate and said applying the at least one liquidis sufficient to lower a force of adhesion between the particulate andthe surface.